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I have two motors (looking like this one) connected in parallel and attached to a 9 V battery; initially motor A is switched on and Motor B is switched off. When I turn on motor B, motor A's speed slows down - and when I turn motor B off, motor A speed is back to previous value.

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How do I prevent this behaviour, i.e. what should I do to keep motors running at same speed no matter if the other motor is on or off?

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    \$\begingroup\$ Use a more powerful battery \$\endgroup\$ – PlasmaHH Jan 20 '17 at 13:27
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    \$\begingroup\$ It's the battery's internal resistance. When you draw more current, the battery voltage drops. To solve this, use a better battery so that the voltage drop will be smaller or use a voltage regulator circuit to make a stable voltage, for example 7 V. \$\endgroup\$ – Bimpelrekkie Jan 20 '17 at 13:28
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    \$\begingroup\$ 6 AAs or even AAAs would have a noticeably lower internal resistance -- something like half assuming the same chemistry. Alkalines are better than zinc-carbon, and NiMH rechargeables are better than alkalines. \$\endgroup\$ – Chris H Jan 20 '17 at 15:46
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    \$\begingroup\$ If the conductors are not sized large enough, they could also be inducing a voltage drop when more current is pulled through them. \$\endgroup\$ – Steve Jan 20 '17 at 20:35
  • \$\begingroup\$ One possibility is to use a buck/boost dc-to-dc converter configured for 9V in and 9V out (or whatever the voltage happens to be when one motor runs off the battery, maybe more like 7 or 8). That will compensate for the voltage drop by drawing more current. However, the increased current draw will cause increased voltage drop. It might be a losing game -- the battery may just not be capable of putting out twice as much energy as one motor draws from it. Plus, the converter won't be 100% efficient, so that hurts you too. \$\endgroup\$ – David Schwartz Jan 20 '17 at 23:51
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Use a better power supply than a 9V battery- one that does not change output voltage significantly regardless of the number of motors. You could also add a regulator.

For example, a 12-volt SLA (Sealed lead-acid) battery with a suitable regulator for the currents involved would be almost perfect. Even without the regulator it would be pretty good (provided your motors were happy enough with the higher voltage).

The problem you note will only get (much) worse as the battery is depleted and its internal resistance rises.

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    \$\begingroup\$ Suitable regulator for motor currents aka motor controller. \$\endgroup\$ – iheanyi Jan 20 '17 at 23:50
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  1. use a beefier battery or a power supply;
  2. use a motor that consumes little current;
  3. use a separate power source to power the motors. ...
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In case you wan't to keep the circuit simple (no regulator) you should look into the C-rating of the used battery. There are some LiPo for drones which can deliver around a 1kW in power which is hopefully enough for your two motors.

The C-rate is a measure of the rate at which a battery is being discharged. It is defined as the discharge current divided by the theoretical current draw under which the battery would deliver its nominal rated capacity in one hour.[32] A 1C discharge rate would deliver the battery's rated capacity in 1 hour. A 2C discharge rate means it will discharge twice as fast (30 minutes). A 1C discharge rate on a 1.6 Ah battery means a discharge current of 1.6 A. A 2C rate would mean a discharge current of 3.2 A. Standards for rechargeable batteries generally rate the capacity over a 4-hour, 8 hour or longer discharge time. Because of internal resistance loss and the chemical processes inside the cells, a battery rarely delivers nameplate rated capacity in only one hour. Types intended for special purposes, such as in a computer uninterruptible power supply, may be rated by manufacturers for discharge periods much less than one hour.

Straight from Wikipedia

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The simplest solution is to keep the motors isolated. This means using a power supply/battery for each motor. This way you can turn each motor on or off, without affecting the other motor.

As to keeping the motors running at the same speed, your control circuitry could become very complicated, depending on the "degree of accuracy" you want the speed to be the same.

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